Multiport - multipoint digital data service

ABSTRACT

In a Digital Data System (DDS) a method and apparatus for providing multipoint multiport service. Differences in time delays through the network are compensated by an alignment training wherein the delays from each remote station are measured at a Central station. The Central station determines how much delay to insert at each remote station and then instructs the remote stations to insert the delays. This allows all inbound data to be aligned in time so that when the data are combined within the network by the Multipoint Junction Units (MJU) data errors are not produced. Aligned frames are combined in the MJU operating in data mode in a logical AND operation so that marks transmitted by inactive ports are combined with data from active ports to produce a composite signal which is passed to the Central DSU.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to the field of multiplexed digitaldata communications. More particularly, this invention relates to anaccess device for a digital network such as a digital service unit (DSU)for interconnecting a plurality of terminal devices running differentapplications and permitting them to share a single transmission channelin a multipoint multiport environment.

2. Background of the Invention

The term Digital Service Unit (DSU), as used herein, may in general alsoembrace combined Digital Service Unit/Customer Service Units (CSU),CSU's, or similar digital network access devices as will be appreciatedby those skilled in the art. It may also embrace similar devicesoperating in digital data networks.

There are many business environments wherein multiple transmission linesare used to carry data to and from various terminal type devices.Typically, the total average data rate for these devices is less thanthe capacity of a single digital transmission line. Separate lines areoften used because of protocol incompatibilities, separate applicationsbeing simultaneously run, gradual evolution of a communication network,connection to more than one Central computer, etc.

One typical example of such a system is that of a bank or otherfinancial institution wherein at a single physical location there existsone or more terminals for use by tellers, terminals used by loanofficers, accountants and the like for running other financialapplications and automated teller machines (ATM). Another example isthat of the retail industry wherein point of sale terminals (POST),credit verification terminals and accounting terminals may all useindividual transmission facilities. Each of these may use their owndedicated analog or digital leased transmission lines which are notfully utilized. Conversion to a single digital multipoint multiportcircuit may be more cost effective in many cases. Even if thetransmission lines are fully utilized, conversion to a higher rate DDSservice using a multipoint multiport circuit may result in substantialtelecommunications cost savings.

When analog data modems are used, various techniques have been devisedto facilitate sharing of transmission facilities and thus reducetelecommunications costs. For example, there are data modems whichutilize frequency division multiplexing to divide a single transmissionline into several channels. An example of such a scheme is shown in U.S.Pat. No. 4,335,464 to Armstrong et al. A time division approach formodems has also been proposed in European Patent Application number88304437.2 published Nov. 23, 1988 under publication number 0292226.Another approach for modems has been described in U.S. Pat. applicationSer. No. 07/355,521 assigned to the assignee of the present inventionand incorporated herein by reference.

In order to achieve higher reliability in data communications at higherspeed, many users are converting to all digital networks such as DDSnetworks. Multipoint (or Multidrop) circuits in DDS networks usemultipoint junctions units (MJU's) or similar digital bridging devicesto combine inbound data from each of the remote units.

An MJU (in a DDS network) allows two different modes of primary channeloperation in the inbound direction. The mode is set in DDS-II by thecontrol bit and in DDS-I by a bipolar violation sequence. In the firstmethod (data mode), all remote stations transmitting to the Centralstation keep the primary channel in the data mode even if it has noprimary channel data to transmit. In place of primary channel data, theRemote stations simply transmit all marks (all data bits set to logicones). In this mode of operation, the MJU combines the data bits fromdifferent drops using the equivalent of a logic AND operation so that ifany station transmits a zero, a zero will be passed to the Central.Otherwise, a logic one will be sent to the Central. This is the mode ofoperation which is used for the present invention.

In the second mode of operation (the control mode), the remote stationskeep the data channel in the control mode by sending control mode idle(CMI) when there is not data to sent. In this mode, the remote stationswitches to data mode only when it has actual data to send. When the MJUreceives a control code on any of the drops, it internally forces thedata bytes from those drops to all marks (ones) prior to providing thelogical AND bridging process. If all drops are inactive, the CMIsequence propagates to the Central. This second mode generally has theadvantage that the CMI sequence can be used to distinguish between anactive channel and an inactive channel thus providing DCD (Data CarrierDetect) control. This second mode of operation is conventionally thepreferred mode of operation of a DDS network for multidrop operation ina DDS network.

In the case of DDS S/C, the MJU is also responsible for detectingsecondary channel activity from a drop in order to bridge it with theprimary channel data sent to the Central. In this case, however, thedesign of the MJU permits only one active secondary channel and ignoresany other secondary channel activity from other drops.

For the preferred embodiment of the present invention, the network, andthus the access devices and MJU's, are used in the data mode rather thanthe control mode so that any remote station which is not transmittingdata transmits all marks. Although using this mode does not provide theadvantage of allowing simplified DCD control, it provides a convenientmechanism for permitting the alignment process of the present inventionto be performed and multiport multidrop service to be provided.

The MJU's operating in the data mode basically perform a digitalbridging function analogous to a logical AND operation on the primarydata of the active channels in order to combine the data from thedifferent points or drops in the circuit. This function may variously bereferred to herein as an AND function, digital bridge function or MJUfunction synonymously and should not be strictly limited to the DDSdefinition of an MJU. The present invention is applicable to any digitalnetwork using similar digital bridging techniques. The MJU may be eitherembedded in the network, for example as part of a digital crossconnectsystem (DCS), or may be in the form of an MJU plug in card as will beappreciated by those skilled in the art.

Simple multi-point operation is contemplated by the DDS serviceproviders and described in their various specifications. In simplemultipoint operation, data from each remote does not need to be alignedin time since only one remote is polled by the Central (and thereforecapable of transmitting) at any given time. However, time alignment ofinbound data traffic may be required in some situations of multipointmultiport operation. For example, in multiport multipoint operationseveral remote sites may be polled simultaneously on different ports bytheir respective applications running at the Central site at any giventime. Thus, the remote could transmit simultaneously during a portion oftheir inbound response to the poll if there is no time alignment ofinbound frames. Since this would result in data errors, appropriate timealignment should be obtained.

More detailed technical information regarding the various Digital DataSystems may be obtained in the various technical specificationspublished by AT&T and other digital service providers for their digitaldata systems (e.g. AT&T Communications Technical Reference PUB 62120,1984). Additional information is also available in U.S. Pat. No.4,745,601 to Diaz et al, which is incorporated herein by reference.

The present invention provides a cost effective method and apparatus foraccomplishing the multiport multidrop function in a digital network suchas the DDS service provided by AT&T and provides for alignment of theinbound frames from remote DSU's. The present invention takes advantageof the characteristics of the digital bridging function to properlycombine inbound multiport data. The present invention details a novelmethod to ensure the integrity of inbound TDM data from various drops ofa DDS multidrop multiport network. This alignment method ensures thateach bit transmitted from each remote arrives at the Central DSUproperly mixed. Thus responses sent by different remote terminalsconnected to different remote DSU's can be kept in their appropriate TDMslots with respect to other neighboring TDM slots and not allowed tointerfere with one another.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an improvedmultiplexed DSU for use in multidrop multiport environments.

It is a further object to provide a method for establishingsynchronization between the various remote DSU's and the Central DSU onsuch a multiport multidrop network.

It is a further object and advantage of the present invention to takeadvantage of existing characteristics of widely available digital datanetworks to provide multipoint multiport service.

It is an advantage of the present invention to provide multiportmultipoint communication in a digital network.

These and other objects and advantages of the invention will becomeapparent to those skilled in the art upon consideration of the followingdescription of the invention.

In one aspect of the present invention, a synchronous digital multiportmultipoint communication system includes a digital network fortransporting digital data bits between a plurality of locations. Aplurality of remote stations are coupled to the network. A Centralstation is also coupled to the network, the Central station transmittingoutbound data from the Central station to the plurality of remotestations via the network. Transmission times of the plurality of remotestations are aligned to compensate for differences in delay through thenetwork from the remote stations to the Central.

In another aspect of the present invention, a synchronous digitalmultiport multipoint communication system includes a Digital DataService (DDS) network for transporting digital data bits between aplurality of locations, wherein the digital network combines data fromthe plurality of remote stations using a logical AND operation carriedout in a Multipoint Junction Unit (MJU). A plurality of remote stationseach including a Digital Service Unit (DSU), are coupled to the network.A Central station including a DSU is also coupled to the network, theCentral station transmitting outbound data from the Central station tothe plurality of remote stations via the network. Frames are establishedaround digital signals transmitted from the remote stations to theCentral station and from the Central station to the remote, the framesincluding a synchronization pattern and a slot dedicated to carryingmultipoint polling commands. Access to transmission over the network bythe remote stations is controlled by polling. Transmission times of theplurality of remote stations is adjusted to compensate for differencesin delay through the network to the Central by measuring an amount ofdelay associated with each the remote station, comparing the delay witha reference delay, and transmitting a representation of the delay to theremote station so that the remote station delays future transmissions byan amount determined by the representation of the delay.

In another aspect of the present invention, a method for providing timealignment of frames of digital signals transmitted from a plurality ofremote stations to a Central station, includes the steps of:establishing a reference time delay for arrival of a reference frame atthe Central station; determining an amount of time adjustment relativeto the reference time delay required to align a remote station'stransmitted frames with the reference; and introducing the amount oftime adjustment prior to transmissions from the remote station to causeframes transmitted by the remote station to arrive at the Centralstation in time alignment with the reference frame.

In another aspect of the present invention, a digital data network, amethod for compensating for time delays in the network so that datatransmitted from a plurality of remote stations in the network arrive ata predetermined location in time alignment, includes the steps of:commanding all of the remote stations to enter an idle state; having aremote station transmit a signal; measuring a difference in time delaybetween receipt of the transmitted signal and a reference time;adjusting a transmission time for transmissions from the remote stationso that the measured difference in time delay is compensated.

A digital network access device, according to one embodiment of theinvention, for providing multiport multipoint communication includes afirst interface for interfacing to a plurality of Data TerminalEquipment (DTE) devices and a second interface for interfacing to adigital network. A framing circuit arranges data bits from the pluralityof DTE devices into a data frame for transmitting to a Central location.Adjustment of the position in time of the data frame is made to alignwith a periodic reference time.

In another aspect of the present invention, digital network accessdevice for providing multiport multipoint communication a receiver forreceiving a framed signal from a remote network access device over adigital network. A measurement of an arrival time for the framed signalfrom the remote network access device in relation to a reference timeris made and a command is transmitted to the remote network access deviceinstructing the remote network access device to adjust its timing sothat the arrival time of future framed signals transmitted by the remotenetwork access device arrive in time alignment with the reference timer.

In another embodiment, a multiport multipoint digital communicationsystem includes a data network including at least one digital bridgingdevice which combines digital input signals into a composite signal withan AND operation. A central site network access device receives thecomposite signal. A plurality of remote site network access deviceslocated at a corresponding plurality of remote sites transmit alignedframes of digital signals to the data network. A multiplexer providesmultiplexing of signals from a plurality of Data Terminal Equipment(DTE) devices at each of the remote sites to respective ones of theremote site network access devices.

In another aspect of the present invention, a method of communicating ina multiport multipoint digital network having digital bridges whichperform an AND function on signals applied to inputs thereof, thenetwork using a frame to arrange digital mark and space signals fortransmission over the network, includes the steps of: transmitting apoll message from a first network access circuit to a second networkaccess circuit, the poll message including an address of the secondnetwork access circuit; at the second network access circuit, detectingthe address; and transmitting a sequence of spaces from the secondnetwork access circuit over the network, the sequence of spaces beingpositioned in time to overlap any marks transmitted by other networkaccess circuits.

In another aspect of the present invention, a method of aligning inboundframes of digital signals bound for a Central site from remote sites,includes the steps of: at the Central site, issuing a global command forall remote sites to transmit marks in each frame location; commanding afirst remote to transmit a frame containing a predetermined pattern;establishing a reference time at the central based upon time of receiptof the predetermined pattern; commanding a next remote to transmit thepredetermined pattern; measuring a relative delay in receiving thepredetermined pattern from the next remote; commanding the next remoteto adjust its transmission time by an amount which causes transmissionsfrom the first and next remote to arrive at the Central site in timealignment.

In another aspect of the present invention, a method of aligning inboundframes of digital signals bound for a Central site from remote sites,includes the steps of: (a) performing an initial alignment process sothat frames transmitted from each the remote site arrive at the Centralsite in time alignment; (b) selecting a remote site from a polling list;(c) determining if the remote is properly aligned; (d) if the remote isproperly aligned, selecting a next remote from the polling list andrepeating step (c) for the next remote; (e) if the remote is notproperly aligned in step (c), correcting alignment of the remote andthen going to step (b).

According to an embodiment of the present invention, in a Digital DataSystem (DDS) a method and apparatus for providing multipoint multiportservice. Differences in time delays through the network are compensatedby an alignment training wherein the delays from each remote station aremeasured at a Central station. The Central station determines how muchdelay to insert at each remote station and then instructs the remotestations to insert the delays. This allows all inbound data to bealigned in time so that when the data are combined within the network bythe Multipoint Junction Units (MJU) data errors are not produced.

Another method of the present invention of combining data from aplurality of multiport remote stations for transmission to a Centralstation through a digital network, includes the steps of: arrangingsignals for transmission to the Central station from the remote stationsinto aligned frames; assigning positions in the frames to each port ofeach remote such that the corresponding ports at each remote areassigned the same frame positions; at an active port of one of theremotes, transmitting data bits in the assigned position for the port;at an inactive port at each remote corresponding to the active port,transmitting all marks in the assigned position in the frame for theport; combining the marks with the data bits in a digital bridge with anAND function to form a composite frame; and transmitting the compositeframe to the Central station.

A method of providing multipoint multiport communication service in adigital network according to the invention, includes the steps of:receiving a first frame containing data in a predetermined time slotdesignated for use by a first port from a first network access device,the first port of the first network access device being an active port;receiving a second frame from a second network access device containingall marks in the predetermined time slot designated for use by the firstport of the second network access device, the second frame aligned intime with the first frame and the first port of the second networkaccess device being an inactive port; combining bits in the first andsecond frames by a logical AND function to produce a composite frame;and transmitting the composite frame to a third network access device.

Another method of providing multipoint multiport communication servicein a digital data network, includes the steps of: receiving a firstframe containing data in a predetermined time slot designated for use bya first port from a first multiport remote digital service unit, thefirst port of the first multiport remote digital service unit being anactive port; receiving a second frame from a second remote multiportdigital service unit containing all marks in the predetermined time slotdesignated for use by the first port of the second remote multiportdigital service unit, the second frame aligned in time with the firstframe and the first port of the second remote digital service unit beingan inactive port; combining bits in the first and second frames by alogical AND function in a multipoint junction unit to produce acomposite frame; and transmitting the composite frame to a Centraldigital service unit.

The features of the invention believed to be novel are set forth withparticularity in the appended claims. The invention itself however, bothas to organization and method of operation, together with furtherobjects and advantages thereof, may be best understood by reference tothe following description taken in conjunction with the accompanyingdrawing.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a block diagram of an example system utilizing the presentinvention.

FIG. 2 illustrates a delay mechanism by which misalignment may occur ina DDS network.

FIG. 3 illustrates misalignment of the frames at the remote units.

FIG. 4 is a timing diagram illustrating the misalignment or a framingpattern in the system of FIG. 3.

FIG. 5 shows a flow chart of an overall alignment process according tothe present invention.

FIG. 6 illustrates proper alignment of the frames at the remote units.

FIG. 7 is a timing diagram illustrating the proper alignment of theframing pattern in the system of FIG. 6.

FIGS. 8A and 8B describe the alignment process of the present inventionin greater detail.

FIG. 9 illustrates the process of stretching a frame to achievealignment.

FIG. 10 describes the process of re-alignment of a single misalignmentremote station.

FIG. 11 is a functional block diagram of a multipoint multiport DSUaccording to the present invention.

FIG. 12 is a functional block diagram of the multipoint multiportprocessor of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Turning now to FIG. 1, there is shown a simple network used toillustrate the overall operation of the present invention. The networkincludes a Central site Multiport DSU 10 (a DSU incorporating a timedivision multiplexer and circuitry to be described later) which isconnected, for example, to a plurality of host computers through a FrontEnd Processor FEP (not shown) via ports I-V. Each host computer (notshown) may be running a different application program, or in a similarscenario, two or more application programs (e.g. Automated Tellersoftware, accounting software, etc.) may be simultaneously andindependently running on the same computer. In this example, five suchapplications (I-V) are shown as applications 12, 14, 16, 18 and 20. Forpurposes of this discussion the term "application" is intended toembrace each of the above possible scenarios and variations thereof. Forpurposes of this discussion the terms "inbound" and "outbound" will bewith respect to a Central site.

On the other side of Central site DSU 10 is a DDS or similar digitalnetwork 22 having an outbound path 24 and an inbound path 26 withrespect to the Central site DSU 10. Recall that throughout thisdiscussion, the terms inbound and outbound are always referenced to theCentral site DSU 10 unless clearly indicated otherwise. For purposes ofillustration, assume that the network 22 provides 38.4 Kbps DDS witheach application being allocated 7.2 Kbps of primary channel bandwidth(six frame slots as will become clear later). Of course, this rate ismerely illustrative of the present invention and is not intended to belimiting.

A plurality of remote site DSU's are attached to the network 22 at aplurality of remote locations by using the services of one or moreMultipoint Junction Units (MJU) or digital bridges within the network(not shown explicitly). In this example, four such DSU's are shownattached to the network 22 with a first such remote site DSU 30 attachedto the network 22 at a first remote location. Remote site DSU 30 iscoupled to five terminal devices 32, 34, 36, 38 and 40 which communicatewith applications 12, 14, 16, 18 and 20 respectively. The terminaldevices may be, for example, computer terminals, point of saleterminals, credit verification terminals, automated teller machines,etc.

A second remote site DSU 46 is also coupled to the network 22 to providenetwork services to five remote DTE's 52, 54, 56, 58 and 60 whichsimilarly communicate with applications 12, 14, 16, 18 and 20respectively as shown. A third remote site DSU 66 is also coupled to thenetwork 22 to provide network services to terminal devices 72, 74, 76,78, and 80 as shown. A fourth remote site DSU 86 is also coupled to thenetwork 22 to provide network services to terminal devices 92, 94, 96,98, and 100 as shown. Of course, more or fewer such remote site DSU'smay be dropped along network 22 in the present invention.

In a more conventional simple multipoint arrangement for providing theabove communications, it would be possible to provide communicationbetween DTE devices 32, 52, 72 and 92 and the application (e.g. host) 12using a single multipoint DDS circuit or similar analog circuit. Inorder to provide connections for the remaining applications, a separateDDS or analog circuit would have to be provided. That is eachapplication would require its own circuit. Obviously, if it werepossible to reduce the number of DDS or analog circuits required, therewould be potential for substantial cost savings. By more fully utilizingthe bandwidth available on a given DDS circuit, and / or by using asingle higher rate digital service, many cost saving opportunities existby using the present invention.

As previously explained, multipoint circuits in the DDS network usemultipoint junctions units (MJU's) to combine the inbound data from eachof the remote units. The MJU's basically perform a logical AND functionon the primary data of the active channels in order to combine the datafrom the different drops when operating in the data mode as in thepresent invention. This principal is equally applicable to other digitalnetworks using similar digital bridging functions. In simple multipointoperation, data from each remote does not need to be aligned in timesince only one remote is polled by the Central at any given time. Whenthe polled remote completes transmission, the next remote is polled andso on in accordance with a polling list or polling table stored at theCentral site.

In a multiport multidrop operation several remotes may be polledsimultaneously on different ports at any given time since theapplications on different ports may not be aware of the pollingcondition of the other ports. This can cause collisions and errors. Whensecondary channel is being used in a multidrop configuration it may beactive from a drop other than a drop that is currently being polled fromthe Central, causing more than one remote station to be activesimultaneously. For these reasons, the aggregate channel frame from eachone of the remotes should be aligned at the input of each MJUencountered in order for the inbound data to be combined correctly atthe MJU(s) and arrive at the Central station at its proper timeinterval.

The operation of the Multipoint Junction Unit and the misalignmentproblem addressed by the present invention may be better understood byreference to FIG. 2 which illustrates one mechanism wherebymisalignment, as shown in FIG. 3, can occur. In this figure, more, butnot complete, detail of the network 22 is shown to illustrate the pathtaken by the inbound data traffic. In this sample network, two MJU's 110and 112 are involved. Additionally, each DSU 10, 30, 46, 66 and 86interfaces with an Office Channel Unit (OCU) respectively designated116, 118, 120, 124 and 126.

The signals passing through the network toward the Central site arecombined by the MJU's 10 and 12 which may be thought of simply aslogical AND gates for purposes of this discussion since the network isoperating in the data mode. Various delays may be attributed to variousconnections (lines) and circuits in the network for purposes of modelingthe delay as shown. In most cases, the delays can be assumed to be aninteger multiple of the bit time due to the elastic buffers of thesynchronous network. The delay from the input of OCU 116 to the input ofDSU 10 is shown as d1 and the delay from the input of MJU 110 to theinput of OCU 116 is shown as d2. The total delay d1+d2 is common to allinbound communication in this illustrative network.

Delay d3 is attributed to the line from the output of OCU 118 to theinput of MJU 110, while delay d4 is the delay from the output of DSU 30to the output of OCU 118. Thus, the total delay from DSU 30 to DSU 10 isd1+d2+d3+d4 =dBA.

In a similar manner, the delay associated with the line from the outputof OCU 120 to the input of MJU 110 is shown as d5. The delay from theoutput of DSU 46 to the output of OCU 120 is shown as d6. Thus, thetotal delay from DSU 46 to DSU 10 is d1+d2+d5+d6=dCA. Depending upon theline lengths and thus the delays d3, d4, d5 and d6, etc., this delay dBAmay or may not equal dCA. The MJU 110 combines the data from DSU 30 withthat of DSU 46.

The delay from the input of MJU 112 to the input of MJU 110 is given asd7. The delay associated with the connection from OCU 124 to MJU 112 isgiven as d8. The delay from the output of DSU 66 through the output ofOCU 124 is shown as d9. Thus, the total delay from DSU 66 to DSU 10 isgiven by dDA=d1+d2+d7+d8+d9. In a manner similar to that explainedabove, the delay from DSU 86 to DSU 10 is given as dEA=d1+d2+d7+d10+d11.

Each of the above delays are, of course, inbound delays. Correspondingoutbound delays are not a problem since outbound data are broadcast toall remote DSU's.

The four delays dBA, dCA, dDA and dEA may differ significantly from oneanother due to the various paths taken to the Central DSU 10. For simplemultipoint operation, this is not critical since only one remote DSU ispermitted to transmit at any given time, as explained previously. But,since the MJU's operate in a manner similar to a logical AND gate in thedata mode, any such misalignment of the inbound data in a multipointmultiport environment can result in data errors.

At any one time, only one remote DTE associated with a particularapplication can be sending inbound data in the time slot or slotsallocated to that particular application. This is because the Centralpolls the remote DTE's one at a time for a given application. Theremaining remote DTE's send idle data bits in the form of all marks inthe application's designated slot or slots. These idle data bits arethen combined with the data bits from the polled DTE to produce acomposite signal in which only the polled DTE signal is passed to theCentral.

Simultaneously, the same thing may be occurring with other applicationson the same network so that the respective time slots allocated to thoseapplications are occupied by data from one remote DSU and idle bits fromthe others.

To maintain alignment for multipoint multiport requires that the delayfor the DDS data bits remain constant through the DSU and the DDSnetwork. If the delay changes in a section of the inbound data path,then the relationship of the Central unit's receive framing to itsframing reference can be lost and errors can occur.

Consistent data delay through the network is assured while the networkis in data mode and no situations causing errors occur. The specifiednetwork error rate for DDS can include some "timing slips" which couldcause loss of framing alignment. The total error rate for DDS isspecified, but the portion of this attributable to "timing slips" is notspecified. Compensation for "timing slips" can be accomplished by theresynchronization methods described later.

FIG. 3 illustrates misalignment of the frames at the remote units in asimplified example network. In this example, a Central DSU 170broadcasts multiplexed data to four remote DSU's 172, 174, 176 and 178through an MJU 180. The sequence of bits shown at the left of eachremote DSU represents one possible sequence of misaligned data where therelative position of each bit represents its time relationship with thecorresponding bits. For the outbound direction, the Central simplybroadcasts the data. The inbound direction is more complex, however,since there is a possibility that the data from the various remote unitsmay have different delays associated with their path back to the Centralas illustrated in FIG. 2. This misalignment can result in collisions anddata errors if not corrected. In order to prevent this, a method ofcompensating for the different delays in receipt of the synchronizationpattern transmitted by each remote DSU is needed. This data misalignmentis also shown in the timing diagram of FIG. 4 illustrating the erroneousdata appearing at the MJU output as a result of the AND gate operationof the MJU.

If left uncompensated, the AND operation of the MJU 180 operating in thedata mode causes a logic one to be transmitted whenever all inputs ofthe MJU receive a logic one. If any one or more stations transmit alogic zero, the output of the MJU will be a logic zero. As shown in theexample of FIG. 3, the misalignment of the data causes four DSU'stransmitting the same pattern with a total of four zeros to be combinedto generate a data stream with nine consecutive zeros, an obvious errorcondition. Assuming that the transmission from DSU 172 represents propertiming, the composite signal shows a total of five errors in dataresulting from misalignment.

The present invention takes advantage of the fact that in a DDS system,the delay through the network remains fixed when all channels are keptactive (except possibly when an occasional timing slip occurs). Thisfact allows the Central to align each remote independently so that thecombined data stream from the MJU is properly aligned. The multidroppoll field (MP POLL) of the framing scheme of the present invention isused to send control information and confirm the integrity of theaggregate channel. After an initial alignment process is complete, theintegrity of the alignment is maintained by systematically polling eachremote unit. If a response to the poll is not properly received by theCentral, it is possible that the remote unit has lost its framealignment. At this point the remote unit's frame is realigned if theCentral determines that the remote unit is present and is capable ofrealignment.

The process of the present invention basically involves operating thenetwork in the data mode and synchronizing the total DDS multiportmultidrop network such that the differences in delay as measured byreceipt of a transmitted frame from each remote DSU with relation to areference stored in the Central DSU are compensated. This isaccomplished by measuring the relative delay and making an adjustment(inserting a delay) in transmit time for future frames at each remoteDSU to ensure that the transmitted signal arrives at the Central MJUaligned in time.

The Central MJU(s) then essentially combine(s) the signals from all ofthe remotes by a logical AND process in accordance with standard datamode operation. With proper time alignment, the data will be properlytransmitted to the Central DSU. With this alignment accomplished, theCentral then monitors the time alignment of frames transmitted from theremote stations to determine if a slip or timing change has occurred. Ifso, a correction of the alignment of from one to possibly all of theremote stations in the network is carried out.

This alignment process taking advantage of the network characteristicswhen operating in the data mode, is a basic premise of the multiportmultipoint system for DDS. If all of the synchronization patterns fromall remote DSU's can be made to arrive at the Central DSUsimultaneously, then the remotes are aligned.

The flow chart of FIG. 5 describes the basic alignment and realignmentprocess used in the present invention in general terms starting at 200.At step 204 the Central DSU is powered up. Control then passes to step206 where an initial alignment process is carried out to obtainalignment of inbound transmission from all remotes. Next, at step 210,the alignment of a remote (preferably the first remote on the CentralDSU's polling list) is inspected to determine that the alignment of thisremote is intact. If it is properly aligned at 212, control passes to216 where the Central increments to the next remote station and controlreturns to step 210 where alignment of this next remote station isinspected.

If any remote is determined to not be properly aligned at 212, controlpasses to 220 where a correction is made, if possible, to the remotethat is misaligned. If it appears that more than one remote station isinvolved (e.g. power failure at a remote facility or network problem) acomplete realignment is performed for the entire system at 220. Controlthen returns to the monitoring step 210.

The measurement process for measurement of the actual delays is carriedout by comparison of receipt time in the Central DSU of the framingpattern with a frame and bit counter within the Central DSU. Thiscounter keeps track of the incoming frames from the remote stations. Forexample, if a frame is 144 bits long, the counter will count from 0 to143 and then reset and repeat the count. The Central station knows thatthe first bit of the frame should arrive when the counter is at 0. If infact the first bit of the frame arrives at count 25, then the delayassociated with the transmitting remote DSU is 26 clock counts. Thisinformation is conveyed to the remote in question who then delaystransmission or stretches it's next transmitted frame for 144-26=118clock counts so that the next frame it transmits will be properlyaligned. This counter can, of course, be implemented either with ahardware counter or in software or firmware.

Once this alignment or re-alignment process has been completed, thetransmission time of the remote units will be properly coordinated sothat transmitted frames as received by the Central DSU and the input ofany digital bridge is properly aligned in time. FIG. 6 illustratesproper alignment of the frames at the remote units in the simplifiednetwork of FIG. 3. This proper alignment of data results in the propercombined data output from MJU 180 as illustrated by the timing diagramof FIG. 7. In this FIG. 7, the framing pattern being transmitted by eachDSU 172, 174, 176 and 178 is properly aligned in time so that no dataerrors are introduced when data are transmitted.

Turning now to FIG. 8, the present alignment process is described ingreater detail. The process of alignment of the network can be initiatedby either power up of the Central DSU or by receipt of a command issuedby a network manager to re-align the network. When one of these eventsoccur at 300, the Central DSU transmits frame bytes with a sync patternin the first time slot and mark hold data (all logic ones) in the otherframe slots to establish the frame boundaries (if the alignment wasinitiated as a result of power up of the Central DSU at 304).

If the alignment was the result of a re-align command at 300, step 304is unnecessary. Control then passes to 308 where remote DSU's get lockedto outbound framing from the Central DSU. At step 312, the Central DSUissues a global command (addressing all remote DSU'S) on the pollingchannel (MD POLL, to be explained later) for all the remote units totransmit marks in every bit of their transmit frame. Then, at 316, theCentral DSU sends a polling message to a first of the remote DSU'scommanding it to send the Sync pattern followed by all marks in theframe. (The first DSU can be chosen, for example, as the DSU having thelowest or highest address.)

Upon receiving the poll in the control channel containing the remoteunit's address at 320, the first remote transmits with the framingpattern in the first time slot of the frame with the rest of the frame'stime slots as all marks. At 324, the Central DSU uses this framingreceived from the first remote DSU to establish a reference time at theCentral for aligning the remaining DSU's. After this, the Centralcommands the remote to return to sending all marks. The Central DSU thensends a polling message to the next remote DSU commanding it to send thesame framing pattern followed by marks as was transmitted by the firstDSU at step 328. Control then passes to 332 where the next remote, uponreceiving a poll in its control channel containing its address,transmits the same pattern which was transmitted by the first DSU to theCentral.

At 336, when the Central DSU receives this framing pattern from thepolled remote DSU, it compares it with the timing of the framing patternreceived from the first DSU. The Central DSU measures how many bit andbyte times this framing pattern differs from the reference establishedby the timing of the first DSU. At 340, the Central DSU sends this delayvalue as a "measured offset" to the remote which was just polled as amessage telling this remote DSU how much delay it should introduce priorto future transmissions so that the timing will align with that of thereference stored in the Central DSU. To implement this delay, the framebit counter in the transmitting DSU is merely adjusted so that one frameis in essence either stretched or shortened.

This process defined by steps 328, 332, 336 and 340 is then repeated asmany times as necessary to find appropriate "measured offset" values foreach remaining remote and transmit such values to all remotes in step344. After the Central DSU is completed establishing the offset for eachremote DSU and transmitting the "measured offset" value to each remoteDSU, the system is then enabled to receive user data at 350.

The addresses which are used to address the various DSU's are preferablythe same addresses as those used for diagnostic and control functionsfor the DSU. This is not to be limiting since any appropriate addressingscheme may be used. In one embodiment, these are addresses compatiblewith the commercially available Racal-Milgo CMS™ series of networkmanagement system such as described in U.S. Pat. No. 4,3385,384 toRosbury, et al which is hereby incorporated by reference.

The transmit frame adjustment is illustrated in FIG. 9 in which 370represents a stream of frames aligned to the reference and 380represents a stream of frames being aligned to this reference. Bothstreams of frames are viewed as received at the Central DSU with time T1occurring prior to time T2, etc. At times T1 through T7 the frames areout of alignment as indicated by the lack of time alignment of thevertical lines representing the frame boundaries. Frame 384 of stream offrames 380 begins at time T6 and is extended by an amount of time equalto T7-T6 which is the amount of difference in the delay between the twostreams of frames. By so extending or stretching frame 384, both streamsof frames 370 and 380 merge into time alignment at time T8 after whicheach frame boundary aligns at times T8, T9 and T10, etc. In a similarmanner, frame 384 could have been reduced in length in manycircumstances so that proper alignment occurred at time T7.

In alternative embodiments of the above system, the exact order of thesteps may be rearranged somewhat or the process may be modified. Forexample, each of the remote stations may be polled and timingmeasurements made prior to sending any correction information in theform of the "measured offset" to the remote DSU's. This might permit areduction in the time required to align a network in which many of theremote DSU's are already in time alignment since their aligned frametimes could be adopted as a reference thus avoiding need to transmitcorrection information to the already aligned remote. Other arrangementswill occur to those skilled in the art.

As discussed briefly in connection with FIG. 5, once proper alignmenthas been attained, data can be transmitted without collision unless oneor more of the remote units loses alignment. This can occur due to poweroutages, network errors, or other factors which might result in a changein the amount of delay at one remote relative to the delay for which thealignment corrected. This problem can be corrected in one of severalways. The initial alignment process of FIG. 8 can be repeated or themethod shown in FIG. 10 can be used in an attempt to minimize the timethat the system is unavailable to the user.

Turning to FIG. 10, a method for alignment correction is described ingeneral terms followed by a more detailed description of the process.This is an ongoing process of monitoring the status of each drop todetermine that each drop is in proper synchronization. By using such acorrection method, timing slips in a single channel or other disruptionsof proper timing can be corrected. The processes starts at 400 and afirst remote station is selected at 402. The Central sends a message (apoll) to the selected remote at 406 requesting that the selected remotereturn an acknowledgement of the message. The Central then listens forthe acknowledgement at 408. If, at 410, the Central receives theacknowledgement and the acknowledgement is error free, the next remoteis selected at 412. When the last remote is checked, the first remote isthe next remote so that the process continuously repeats.

The Central looks for errors in the acknowledgement to determine whetheror not a remote is properly aligned. To allow for communication errors,the Central does not assume that a remote is misaligned or down unlessit has transmitted two consecutive unanswered polls. The Central waitsafter each poll for a predetermined time period to obtain a response. Ifthis time period expires on two consecutive polls or if errors areobtained on two consecutive polls, the remote is assumed to be down ormisaligned.

If a misalignment is detected, control passes to 416 where the Centralcommands all remote stations to transmit all marks. Next, the Centralassures itself that the remote station is present before any steps aretaken to extensively shut down the flow of user data. This is done bythe Central commanding the remote in question to transmit a sequence ofspaces (zeros, in the preferred embodiment it will transmit one frameplus one slot of zeros). This pattern is guaranteed to get through tothe Central if the network is functioning properly since zeros passthrough the MJU without regard for the signals on the other channels dueto the AND operation of the MJU. If this is not received by the Central,the Central can be sure that the remote is not on line.

Then, at 418, a resynchronization of the current remote is initiated bycommanding the current remote to send a sync pattern. At 420, the syncpattern is received by the Central and the delay error is measured withrelation to the reference frame established during the initial alignmentprocedure. The "measured offset" value is then transmitted back to thecurrent remote at 424 and then, at 430, the Central commands all remotesback on line to restore normal operation. Control then returns to 412where the next remote is selected for verification. Other methods mayalso be devised to correct for an error in a single remote station.

The above is, of course, a simplified description of the process. A moredetailed discussion of the realignment process of the preferredembodiment is as follows.

As previously discussed, after the Central has synchronized all theremote stations in the poll table and brought customer data on linethere is still a requirement for synchronization of remote. A remoteunit can be in an unsynchronized state for many reasons. These includeline slips affecting a remote, temporary power failure at a remote,plugging in a new remote to a live network and changing a unit'saddress. Because of this requirement the Central needs to periodicallycheck on the synchronization of all units in the poll table andresynchronize them if needed or remove them from the poll table if theyare no longer present. The Central also needs to check on all theremaining diagnostic addresses to allow initial entry of units into thepoll table as well as re-entry of remote stations into the poll tableafter they were removed because of previous problems.

The Central periodically polls each remote in the poll table, it willalso poll all the other possible diagnostic remote unit addresses. Thispolling is done using an "MD Poll" channel within the multiport frame.In order to provide a quick response time to the known units in the polltable a specific polling order will be used. The polling process startswith the Central sequentially polling each unit in the poll table andthen poling one remote from the remaining address of available legaldiagnostic addresses (256 addresses are available in the preferredembodiment). Next, the Central again sequentially polls each unit in thepoll table and then polls the next diagnostic address. This continuesuntil all of the remaining diagnostic addresses have been polled andthen the polling process starts over. This ensures that only one remoteunit that is not in the poll table is polled between successive polls ofa unit in the poll table.

When the Central polls a unit that is in the poll table it expects anacknowledge message in the MD POLL channel from that remote. The arrivalof this message in the MD POLL channel timing slot of the multiportframe verifies that the remote has proper frame alignment. Up to twosuccessive polls will be issued to the remote to allow for communicationerrors. If after a timeout no reply is received, then this informationshould be used in a integration process to detect problem units, theremote will not be removed from the poll table at this time. Theintegration process should determine that a remote in the poll table hasnot responded to polls for a certain time period and then cause it to beremoved from the poll table. The timeout should be on the order of thetime required for a unit to power-up and lock onto the Central transmitframing. This allows a quick recovery time for remotes that experienceproblems up to the severity of a momentary power interruption. Sincethey are not removed from the poll table, they will receive a pollmessage frequently which allows them to go through the resynchronizationsequence.

A simple timing slip can be corrected by instructing a remote totransmit a predetermined pattern in the MD POLL field of the frame anddetermining if the pattern is either one bit too fast or slow. Thepreferred pattern is a space surrounded by marks. For the preferred fourbit slots, a 1011 or 1101 pattern can be used. The Central can thencheck to see if this pattern is off by a single bit time. If so, theCentral then commands the remote to adjust its timing accordingly. Ifthis adjustment process fails, the other approaches discussed herein canbe used.

After the Central has polled a unit in the poll table and the remotereplies, or 2 polls and timeouts are completed, then polling commencesat the next remote. The Central first tries once more to contact theremote by sending a command to transmit a sequence of zeros to see ifthe remote is present before commencing polling. The remote, uponreceiving a poll to its address, sends a reply in the poll channel if ithas an internal "in-sync" flag set. The "in-sync" flag is used toindicate a condition of proper frame alignment and is set during thealignment or realignment process. If the "in-sync" flag is not set, theremote sends a special sequence to the Central that is guaranteed to bereceived by the Central even if the remote does not have its transmitframing synchronized. This message sequence, for example, consists of Xconsecutive multiport time slot bytes transmitted as all zeros, where Xis one greater than the number of time slot bytes in the multiportframe. The data pump has the responsibility of detecting this message bydetecting X-1 consecutive all zeros bytes. If the Central receives thissequence during the time it is waiting for the reply from the remote itbegins a synchronization procedure for that remote. This begins with theCentral issuing a global command to all remotes to transmit a mark inevery byte slot in their frame. This command should preferably be issuedtwice in two consecutive frames to assure that the command is receivedby all remotes even in the event of a communication error. Upon receiptof this command, data are of course interrupted. This fact iscommunicated to the controller (system control processor 545 of FIG. 11)so that the DTE data ports can be notified that data flow is beinginterrupted.

The Central continues its synchronization session with the remote byissuing a command to the remote for it to transmit one frame with theframing pattern in the first time slot of the frame and with the rest ofthe time slots transmitted as all ones. The Central waits up to aspecified timeout for a frame byte from the remote. If after the timeoutthe frame byte is not received the Central reissues the command once. Ifthe frame byte is not received after a second timeout the pollingcommences at the next diagnostic address in the poll table. In this caseof no response the unit's diagnostic address is passed to an integrationprocess which will cause the unit to be removed from the poll table ifit continues to not respond to requests. If a frame byte is received,the Central measures how many bit and byte times this is off from thereference receive framing.

This "measured offset" value is sent in an adjust (ADJ) message to theremote for it to use to update its transmit framing. When the remotereceives the adjust message it adjusts its transmitter by the amountspecified in the message and transmits all ones in the frame. After theCentral has transmitted the adjust message the Central verifies thesynchronization by sending the remote a command to set its "in-sync"flag and for it to transmit an acknowledge in the MD POLL channel. TheCentral repeats this sequence up to 2 times if timeouts occur due to noresponse. If no response is received from the remote, a message will besent to it to clear its "in-sync" flag. The unit's address will not beremoved from the poll table at this time, but is passed to a backgroundintegration operation. This integration operation is responsible forremoval of the addresses from the poll table if it determines that it isappropriate to do so by virtue of the unit failing to respond apredetermined number of times in a given time period (integrationthreshold). This integration threshold can be determined experimentally.

The remote should integrate the number of failed resynchronizationattempts. If the number exceeds a predefined integration threshold, thenthe remote will automatically squelch itself and generate theappropriate alarms (Maydays). This will prevent a noisy line ormalfunctioning unit from repeatedly bringing down user data.

After the Central is finished with the synchronization session for thatremote it enables user data operation. This is done by sending a globalenable transmitter command called SMD (Switch to Data Mode) from theCentral, twice for reliability, which causes all remote stations withtheir "in-sync" flags set to renew transmission of multiport framing anddata.

A remote is removed from the poll table when there is an indication ofan abnormal condition at that remote. To prevent this remote stationfrom continuously disrupting user data, the remote should preferablykeep statistics on its problems and independently act on them in thepreferred embodiment. It is particularly advantageous for the remote tokeep statistics on how often the Central is trying to align its framing.If the number is excessive (greater than a predetermined threshold) theremote issues a Mayday to this effect to the network management systemand also squelches all transmission. This prevents an intermittentremote from demanding frequent framing realignments which requireinterruption of all network user data.

The operation when a Central polls a diagnostic address that is not inthe poll table is almost the same as described above for a remote thatis in the poll table. There are several differences however. First ofall the Central does not expect a poll reply by the remote in the MDPOLL channel. Rather, it only expects the long sequence of zeros messageif a remote is present at the polled address. Just as for the remoteunits in the poll table, the Central should wait a timeout period, longenough to receive an all zeros message from the remote requestingresynchronization, before it proceeds to poll the next remote. Theremainder of the preferred alignment process is preferably identicalwith one exception. If the Central has successfully aligned the remoteit should also preferably enter the remote address into the poll table.

The present invention can be implemented in either the DDS-I or DDS-II(basic DDS or DDS-S/C) or other compatible or similar synchronousdigital services using outbound broadcast and digitally bridged inbound(i.e. use of an AND function or similar to combine inbound data).Currently, the most prevalent of such systems are the DDS networks, butthis is not to be limiting since the present invention will potentiallywork with other similar systems.

The present invention may be implemented in a number of ways, as will beclear to those skilled in the art. In order to implement the preferredembodiment of the present invention in a DDS network, a frame structureis imposed upon the data as shown in the following Table 1:

                  TABLE 1                                                         ______________________________________                                        SYNC   MD POLL    SEC. CH   D1  D2   . . . Dn                                 ______________________________________                                    

In this embodiment, the frame structure allows for a SYNC field to carrythe synchronization pattern which is used to identify the start of theframe and to maintain synchronization and alignment, an MD POLL fieldwhich makes up the MD POLL channel to carry the polling and alignmentrelated commands, a secondary channel field for use in providingsecondary channel control and diagnostics, and a plurality of datafields for user data. The first field (SYNC) is a sync pattern fieldwhich carries a sync character which marks the start of a frame and isused by the receiving unit to establish frame boundaries. For 8 bitbytes, 01100010 is used, for 7 bit bytes, 0110001 is used and for 6 bitbytes, 011000 is used, but this is not to be limiting. The MultiDropPoll (MD POLL) is used to control and confirm the integrity of the framealignment as will be described more fully later. The Secondary Channel(SEC CH) field is used to provide for a diagnostic secondary channel ormay be used for other purposes. The data fields (D1, D2, D3 ... Dx)contain primary channel data. Of course, those skilled in the art willappreciate that while this arrangement is preferred, it is not intendedto be limiting as other arrangements of the frame are possible. Also,the same framing can be used for point to point multiport arrangements.

The preferred framing is arranged so that the data fields providecommonly used data rates for data communications (e.g. 1200, 2400, 4800,etc.) although this is not intended to be limiting. This is accomplishedby appropriately selecting the byte length and frame length to providefor even division by 1200 bps. Table 2 below shows the preferred framedesigns for some of the various DDS-I services currently available;other arrangements will occur to those skilled in the art:

                                      TABLE 2                                     __________________________________________________________________________                        SEC CH    1200 BPS                                              FRAME                                                                              BITS/                                                                             SEC CH                                                                             RATE DATA DATA   MD POLL                                  SERVICE                                                                             SLOTS                                                                              SLOT                                                                              SLOTS                                                                              (KBPS)                                                                             SLOTS                                                                              CHANNELS                                                                             SLOTS                                    __________________________________________________________________________      56K 140  7   6    2.4  132  44     1                                        38.4K 96   6   4    1.6  90   30     1                                        19.2K 48   6   1    0.4  45   15     1                                         9.6K 24   6   1    0.4  21    7     1                                        __________________________________________________________________________

Although the data channels are expressed in Table 2 in terms of numberof 1200 BPS channels available, it will be clear to those skilled in theart that this should not be construed as limiting the channels to 1200BPS. For example, in the case of 56 K DDS service, 22 channels of 2400BPS or 11 channels of 4800 BPS could also be provided. Similarly, 11channels of 2400 BPS and 22 channels of 1200 BPS could be provided orother variations of the available bandwidth can be devised. Asdisclosed, each byte of the frame represents 400 BPS so that otherbandwidth allocations are also possible.

In the case of DDS-II, a similar framing scheme can be used according toone embodiment. However, DDS-II has its own frame structure imposed bythe network definition. Table 3 below shows the subrate DDS-SC frameformat. In this Table, D1-D6 represents six data bits per frame, Frepresents a framing pattern bit and S/C represents a shared secondarychannel and control bit. The framing pattern used is a repeating 101100pattern. For 56K DDS S/C, the same frame format is used except for theaddition of a D7 bit (seventh data bit after D6 and before F Bit).

                  TABLE 3                                                         ______________________________________                                        D1    D2       D3    D4     D5  D6     F   S/C                                ______________________________________                                    

For each of the DDS services (DDS-I and DDS-II), multiplexing can beaccomplished by a number of different techniques. In general, however,time slots must be allocated in some way to particular channels as withthe divisions shown in Table 2.

In a simpler embodiment of the present invention which still takesadvantage of the network's operation in the data mode, channelallocation and frame alignment, may be done by simply using the DDS-S/Cframe alone without imposing a separate frame structure. In this simplerembodiment, the network automatically provides frame alignment and analignment process is unnecessary. However, this simpler embodiment hasseveral inherent restrictions. By being restricted to the DDS-S/C framestructure, allocation of channels is less flexible than the otherembodiments due to the small frame size and it is more difficult toallocate channels in standardized increments. Also, this embodiment isonly usable for the DDS-S/C service, and their equivalents, which arecurrently not as widely available.

This simpler embodiment does have several important advantages over theother more complex embodiments, however. The implementation is muchsimpler and there is no need for a frame alignment or realignmentprocess since the framing is provided by the system. As an example ofthis simpler system, consider again Table 3. Using DDS-S/C 9.6Kbservice, the frame can be divided into, for example, two 4.8Kbpschannels to make a two port multiport multipoint DSU. For example, bitsD1, D2 and D3 can be assigned to a first port and bits D4, D5 and D6 canbe assigned to a second port. Alternatively, alternating bits can beassigned to alternating ports or any other suitable combination can bemade to provide two 4.8Kbps channels.

In any case, the inactive ports from other remote DSU's transmittinginbound data operate identically to their operation in the otherembodiments described. Namely, they transmit all marks in place of databits while not transmitting data. Thus, the MJU's of the network operateidentically in the data mode by combining data from the various remotesby ANDing the bits together to create a composite signal. Other possibleport combinations will occur to those skilled in the art. Of course,when using this embodiment, the basic limitation in allocation ofbandwidth is that the bandwidth can only be assigned in multiples of onesixth of the service rate for subrate service and one seventh of theservice rate for 56K service due to the number of bits in the servicedefined frame. (Similar limitations would apply to other services usingdifferent frame sizes.)

This limitation in the DDS-S/C embodiment can be overcome to a degree byusing rate adaption techniques. For subrate services, standard datarates can be achieved by assigning the bandwidth in 3/4 bit increments.For example, for 9.6Kbps service three channels could be assigned asfollows. The first channel could be assigned 3/4 of the bandwidth of onebit to obtain a 1200 bps channel. A second channel could be assigned 3/4of the bandwidth of two bits to obtain a 2400 bps channel. A thirdchannel could be assigned the bandwidth of three bits to obtain a 4800bps channel. The remaining bandwidth can be use to provide framinginformation by transmitting data in eight bit bytes. This is illustratedby the following Table 4 illustrating a data pattern for eightconsecutive DDSS-S/C frames. In this table, bits allocated to the threechannels above are designated CH1, CH2 and CH3 respectively.

                  TABLE 4                                                         ______________________________________                                        D1    D2      D3      D4    D5    D6    F    S/C                              ______________________________________                                        F1    F2      F2      CH3   CH3   CH3   F    S/C                              F1    CH2     CH2     CH3   CH3   CH3   F    S/C                              CH1   CH2     CH2     CH3   CH3   CH3   F    S/C                              CH1   CH2     CH2     CH3   CH3   CH3   F    S/C                              CH1   F2      F2      CH3   CH3   CH3   F    S/C                              CH1   CH2     CH2     CH3   CH3   CH3   F    S/C                              CH1   CH2     CH2     CH3   CH3   CH3   F    S/C                              CH1   CH2     CH2     CH3   CH3   CH3   F    S/C                              ______________________________________                                    

The framing bits F1 and F2, which can be for example spaces or a patternother than all marks, are only transmitted when the remote port CHI isactive. Otherwise, marks are transmitted. The Central locks onto theframing pattern F1, for example, to find the beginning of the Channel 1data and to assure that the Central remains locked to the data toperform the rate adaption correctly. Whenever a different remote portbegins transmitting, the Central relocks to the F1 framing pattern.Similar statements apply to CH2 and F2. Channel CH3 requires no framingsince no rate adaption is needed.

For DDS-I, there is no network imposed frame as above for DDS-II.Instead, control information is communicated using bipolar violations.Thus, in order to provide for separate channels in DDS-I, a framestructure is imposed by the DSU upon the data in any suitable manner toappropriately divide up the available bandwidth. In the preferredembodiment, the aggregate channel is partitioned into a plurality ofsequential time slots and a frame is imposed upon the data.

To implement the present invention in a DDS-II network, the presentinvention uses essentially the same scheme as that of DDS-I. In sodoing, the frame structure imposed by the network is simply transmittedwithin the DDS-II frame's data area as required by the network withevery byte of the frame of the present invention occupying a singleDDS-II frame. Thus, each slot of the frame of the present invention willcontain a Frame bit, and a S/C bit which are transmitted to satisfy thenetwork requirements. The secondary channel function provided by thenetwork may be used if desired for it's intended purpose. However, inthe preferred embodiment, additional secondary channel bandwidth isallocated. The preferred frame structure for some of the presentlyavailable DDS-SC services is shown in Table 5 below.

                                      TABLE 5                                     __________________________________________________________________________                        SEC CH    1200 BPS                                              FRAME                                                                              BITS/                                                                             SEC CH                                                                             RATE DATA DATA   MD POLL                                  SERVICE                                                                             SLOTS                                                                              SLOT                                                                              SLOTS                                                                              (KBPS)                                                                             SLOTS                                                                              CHANNELS                                                                             SLOTS                                    __________________________________________________________________________      64K 160  8   14   5.6  144  48     1                                          56K 140  7   6    2.4  132  44     1                                        38.4K 96   6   4    1.6  90   30     1                                        19.2K 48   6   1    0.4  45   15     1                                         9.6K 24   6   1    0.4  21    7     1                                        __________________________________________________________________________

The following Table 6 illustrates the preferred format for the MD POLLfield of the frame of Table 1 which is used to convey the polling andcontrol information used in implementing the present invention. Ofcourse, those skilled in the art will appreciate that this protocol isnot to be limiting since any number of appropriate protocols forconveying the appropriate commands can be used.

                  TABLE 6                                                         ______________________________________                                        U2    U1      OP     D/C    MP3  MP2    MP1  MP0                              ______________________________________                                    

The above format is applicable to any of the DDS-I or DDS-II typeformats. The byte size depends upon the service, e.g.: 8 bits for ClearChannel 64K service, 7 bits for 56K service and 6 bits for subrateservices. The U1 character is available only for the 64K or 56K (eightand seven bit byte) services and the U2 character is available only forthe 64K service (eight bit byte). The MP0-MP3 bits form a four bitnibble (MP nibble) of the protocol's data or command. Two consecutivenibbles make up a byte. The D/C slot is used as a flag to indicatewhether the MP nibble contains a command or data. The OP field is theodd parity of the MP nibble which is used in a conventional manner forerror checking. When the U1 field is available, it is also filled withthe odd parity of the MP nibble. When the U2 field is available, it isfilled with the even parity of the MP nibble. Messages are transmittedby use of a string of MP nibbles to make up a complete message. Eachnibble is referred to below as a field in the command structure.

In the preferred embodiment, each complete message begins with a commandfield CMD having the D/C flag set to zero (indicating a command ratherthan data). There are three types of messages supported: normal message,global message and data message. A global command contains only the CMDfield. A normal message contains a CMD field followed by two addressfields (high and low nibbles of an eight bit address). The format of adata message is a CMD field followed by two address fields as above,followed by a plurality of data fields. A reply field follows forresponse from the remote to the Central.

The following Table 7 types of commands can be issued in the currentform of the above protocol, but others can be added for other purposesas required:

                  TABLE 7                                                         ______________________________________                                        COMMAND      DESCRIPTION                                                      ______________________________________                                        SWI          SWITCH TO IDLE                                                   EFS          ENABLE FRAME SEARCH                                              SAP          START ALIGNMENT PROCESS                                          ADJ          ADJUST                                                           SDM          SWITCH TO DATA MODE                                              MP           MAINTENANCE POLL                                                 SS           SEND SPACE                                                       CIS          CLEAR IN-SYNC                                                    ______________________________________                                    

The SWI command is sent to the remotes to place them in the idle mode tosend all marks to the Central. This command is sent to the Central ateither the beginning of the initial alignment process, after a power oncondition or when subsequent alignments are required. The EFS command issent to one remote requesting it to send frames so that the Central isable to obtain a frame reference. The SAP command is sent to aparticular diagnostic address to instruct the remote to send one framewith all ones in the data field and with a synchronization pattern inthe SYNC field so the Central can find determine the proper "measuredoffset". The ADJ command is sent to a specified diagnostic address toset the "in-sync" flag of the remote and for the remote to adjust thetransmit frame using the offset in the data field of the command.

The SDM command is a global command sent to enable remotes with"in-sync" flags which are set to begin transmitting user data. The MPcommand is used to poll the remotes to verify that they are in alignmentduring the online alignment maintenance process. A reply message is sentby the remote in the MD Poll field, to acknowledge reception of thepoll. A new remote station on the network sends a frame of all zeros inresponse to the MP and is thus identified by the Central. Of course,this all zero frame may corrupt data on the line.

The SS command is used to command a remote to send a frame of all spacesplus one slot time of additional spaces so that it can be positivelyidentified as a response by the Central. Although the frame data fieldscan possibly contain all zeros, the frame sync pattern cannot so thispattern of all zeros is sure to be recognized by the Central as aresponse. The CIS command is used by the Central to command a remote toclear its "in-sync" flag. The CIS command can be either a normal commandor a global command.

While the above protocol for communication using the MD Poll slot issuitable for implementation of the present embodiment, it will be clearto those skilled in the art that other protocols may be equallysuitable. The present protocol may thus be varied without departing fromthe present invention.

Turning now to FIG. 11, a functional block diagram of a DSU operating asdescribed above is shown. The DSU includes a plurality of DTE interfaces502, 504, 506, 508, 510 and 512 for providing suitable attachment of DTEequipment using conventional RS-232 or similar interfacing technology.These interfaces communicate with a multiport processor 516 via a commonbus 520. This multiprocessor interface provides control over bus 520 bydetermining which bus time slots are used by each channel (DTEinterface) for transport of user data into and out of each DTEinterface.

The multiport processor 516, which is preferably implemented using anIntel 80286 processor, passes data to and from a data transport circuit524 which is preferably implemented as an ASIC. Data transport circuit524 provides timing and control functions to the multiport processor 516as well as buffering functions for the data flow. Data passes from thedata transport circuit 524 to a data pump 528 which includes a data pumpprocessor 530, implemented with an NEC V25 processor, and a multipointmultiport processor 532 which communicate via a common bus 535.

Outgoing data from data pump 528 is passed to a conventional customerservice unit (CSU) 540 which also passes incoming data to data pump 528.The CSU 540 serves as an interface to the digital network in aconventional manner.

A system control processor 545 is implemented using an Intel 80188processor and provides high level system control functions to the DSU togenerally oversee configuration and strapping functions, etc. The systemcontrol processor 545 is coupled to a control panel 548 to permit theuser to select operational options as well as strap settings, portspeeds, etc. The system control processor 545 is coupled to a networkmanagement system interface which permits direct communication to anetwork management system such as that described in the above referencedRosbury et al patent. The system control processor 545 is coupled to abus 554 which gives it access to the data transport circuit 524.

The DTE interfaces (502, 504, 506, 508, 510 and 512) implement therequirements of the DTE interface such as synchronous timing for datatransfers and operation of control signals to meet interface standards.The data is transferred between this block and the multiport processor516 by reads and writes to registers in the multiport processor block516. The actuation and monitoring of the DTE control signals is alsoperformed by read and write operations to this block by the multiportprocessor 516.

The multiport processor 516 performs the TDM function on the DTE portdata. It collects data received from the DTE port interfaces and insertsthem into their assigned TDM data slots at the interface to the Datatransport circuit 524. Likewise it extracts the data for each DTE port,from the TDM data at the data transport circuit interface, and writes itout to the assigned DTE interface.

The data transport circuit 524 performs the interface function for dataand control information being transported between the differentprocessors in the system. The interface to the multiport processor 516is a TDM format. This uses a synchronous parallel byte transfer everyTDM time slot. The start of the receive and transmit TDM frames issynchronized to the multipoint multiport processor 532. The interfacesto the system control processor 545 and the data pump 528 are throughread and write registers in the data transport circuit 524. The datatransport circuit thus provides a pathway for user data between the datapump 528 and the multiport processor 516 and a pathway for networkmanagement 550 and control panel 548 information between the systemcontrol processor 545 and the data pump 528.

The data pump 528 interfaces to the network using a TDM frame formatthat contains user data channels as well as network management channelsand channels used to monitor and control the multipoint multiportsynchronization. The data pump is responsible for sending and receivingmultipoint multiport synchronization messages to the data pumps in otherDSU's in the digital network for them to establish and maintain thissynchronization. The data pump 528 does not alter the customer datafields in the TDM frames, it transfers these from/to the multiportprocessor through the data transport circuit 524. The data pump 528extracts / inserts the network management messages into the TDM data fortransport over the digital network. It also transports these messages to/ / from the system control processor by way of writes and reads toregisters in the data transport circuit 524.

The data pump 528 includes a data pump processor 530 implemented with amicroprocessor and a multiport multipoint processor 532 which isimplemented with an ASIC device. The data pump processor 530 isresponsible for higher level tasks such as performing the algorithm toachieve multipoint multiport synchronization. The multipoint multiportprocessor 530 performs the more real-time aspects such as implementingthe TDM frame time slots with counters synchronized to the digitalnetwork.

The system control processor 545 contains configuration information forthe unit such as port speeds, etc. It also monitors the operation ofother parts of the DSU. It translates information from one form toanother when information is passed from one interface to another. Forinstance high level commands received from the network managementinterface 550 are translated to low level actions in the DSU such aswriting a byte to a hardware register to change the speed of operationof an interface port. Another example is where detection of buttonsbeing pressed on the control panel 548 cause corresponding messages tobe displayed on same.

The multipoint multiport processor 532 is shown in greater detail inFIG. 12. This circuit is implemented as a custom integrated circuit inthe preferred embodiment, but this is not to be limiting. The multipointmultiport processor 532 includes a register bank 600. All data, controland status information is passed between the multipoint multiportprocessor 532 and the data pump processor 530 through the registers ofthis register bank 600. They are accessible from the processor 530, viaa microprocessor interface 606, using read and write operations over thecommon data bus 535. A transmit shift register 602 combines the transmitdata, secondary channel and control information into a serial bit streamafter receiving the data from the Tx Buffer and S/C processing block604. Block 604 performs serialization, buffering and sequencing of thesecondary and control information for insertion into the transmit bitsteam.

A scrambler / frame align circuit 608 receives the transmit bit streamfrom the transmit shift register 602. The scrambler portion of 608 isused for LADC applications to limit the energy transmitted on the linesby performing a conventional scrambling function. The frame alignsection of 608 is used in multiport multidrop to adjust the time whenthe transmit frame is started to align it with all other drops.

A Violation generator 610 is coupled to the output of the scrambler andframe align circuit 608 to generate the bipolar violation controlsequences for support of the basic DDS (DDS-I) line format. The outputof the violation generator 610 is fed to a Dual line converter 614. Dualline converter 614 splits the transmit stream into 2 digital outputstreams that will correspond to positive (TXDP) and negative (TXDN)bipolar pulses on the DDS line. These are converted to the bipolarformat prior to transmission over the DDS line.

A frame generator 616 generates the DDS S/C, SDM and multiport multidropframing pattern as required and provides those patterns to the transmitshift register for appropriate combination with the transmit datastream. Frame generator 616 also includes the counter circuit used tokeep track of the proper timing for the transmit frame. This counter isadjusted by the ADJ command from the Central to effect alignment of theremote. The DSU shown in FIGS. 11 and 12 may be either remote or Centraldepending upon strapping configuration. Clock divider 620 divides thetransmit bit clock down to a byte clock that defines the period of thetransmit byte. The number of bits per transmitted byte is configureddepending on the DDS service being used. Clock divider 622 divides thereceive bit clock down to a byte clock that defines the period andboundaries of the receive byte. The number of bits per received byte isconfigured depending on the DDS service being used.

A Transmit Digital Phase-Locked Loop (DPLL) 626 is used in LD typeapplications when the unit is supplying the clocking information for theline. External transmit clock from the DTE can be selected for thesource of the clocking or the DPLL can generate a stable clockinternally. The DPLL is not used when connecting to the DDS networkwhere the clocking information comes from a stable clock sources in thenetwork. This also generates a 1200 Hz reference, that tracks thereceive clock, that is used by other LSI chips in a DSU.

A Receive DPLL 628 is used to derive the receive clock from the receiveddata pulses from the DDS receive line. The receive clock is used toclock in the received data. This also generates a 1200 Hz reference,that tracks the receive clock, that is used by other circuitry in theDSU.

A Dual line converter (receiver) 630 decodes the two incoming signalsthat correspond to positive and negative bipolar pulses on the DDS lineto one serial data stream. The dual line converter 630 then sends thesedecoded signals to a Violation detector 632. This violation detector 632detects violations of the normal bipolar encoding rules found in thereceived data. These are used in the basic DDS service (DDS-I) to passcontrol information to the DSU. The violation detector 632 passes itsoutput on to an unscrambler and frame align circuit 634 which is acounterpart of the scrambler and frame aligner in the transmitter.

A Frame detector 640 in cooperation with the frame align circuit of 634is used to detect and synchronize to the framing methods used in thereceived data. The DDS S/C and SDM framing of the standard DDS servicesare supported as well as the framing for multiport multidrop. The outputof the unscrambler and frame align circuit 634 is passed to a Receiveshift register 644. Here the serial received byte is converted toparallel and the data, secondary channel, control and framinginformation is extracted as required for the service configured for. TheRx buffers / S/C processing block 646 processes the incoming secondarychannel and control information. It separates it and buffers it to theregister bank 600.

In operation, a remote synchronizes its receive frame to the transmitframe of the Central. This synchronization takes place when the remoteis powered up or after it has lost framing from the Central for amultiple number of frames.

The multipoint multiport processor 532 is commanded through the registerbank 600 to start the process of locking to the framing pattern using a"frame search" flag. The multipoint multiport processor 532 will"unlock" from the present byte position it is looking for the frame bytein and start searching the bit stream starting on the next bit position.The multipoint multiport processor 532 scans every byte position in theincoming data stream until it detects a byte that matches the framingpattern in 640. At that time it stops scanning every position andmonitors that byte position at the beginning of the next frame for theframing pattern. When this next frame byte is received, and if itmatches the framing pattern, then the receiver locks to that byteposition and sets appropriate status indicators that framesynchronization has been achieved. If this framing byte does not matchthe framing pattern then the multipoint multiport processor 532 resumesscanning for the framing pattern starting at the next bit position inthe incoming bit stream. After frame lock has initially been achievedand reported in the status register the receiver maintains a lock tothat byte position until commanded to start searching again. After framelock has been achieved the multipoint multiport rocessor 532 monitorseach byte in the frame position of incoming frames for errors andreports these in its status register. The changes of frame locking torandom data is dependent on the frame size and the number of framesmonitored before frame lock is performed.

After frame lock is achieved the controller 530 monitors the frameerrors, integrates them, and commands the multipoint multiport processor532 to start searching for frame again if too many frame errors arefound., Frame lock is one of the qualifications for the data pump to useto report that it is receiving valid data, i.e., for data to go to theDTE ports etc.

The synchronization of all the remote stations in the Central's polltable, after loss of network multiport framing, is described here. Aremote unit that does not have its transmit framing aligned with theCentral's receive framing reference will be referred to as anunsynchronized unit. An unsynchronized unit transmits all ones so as notto interfere with the transmissions of other units. The remote has aflag indicating if it has had its transmit framing aligned to theCentral's receive reference, this flag is referred to as the "in-sync"flag. This flag would be cleared upon power up and under some errorconditions. At the beginning of the initial synchronization procedurethe Central first transmits frames with all slots transmitted as marksfor long enough to allow all remotes to lock to outbound framing. Thisis needed in the case where the Central unit has just powered up and theremotes are not locked onto outbound framing and therefore can notextract the polling channel messages. Next the Central issues a globalcommand on the MP Poll channel for all the remote stations to cleartheir "in-sync" flag and to transmit marks in every bit of theirtransmit frame.

In order to initially synchronize all the units in the poll table theCentral conducts a synchronization session with one unit at a time, oncefor every unit in the poll table. For each synchronization session theCentral first transmits an EFS command to the unit using its diagnosticaddress. The Central expects a frame byte in every frame to be receivedfrom the remote for the Central to lock to the first remote's framingwhich is used as a reference.

Upon receiving an EFS command, in the control channel, containing itsdiagnostic address the remote enables the transmitter to transmitcontinuously with the framing pattern in the first time slot of theframe and with the rest of the time slots transmitted as all ones. Ifthis is the first remote being aligned, the framing received by theCentral will be used by the Central as the reference for aligning allremote stations. The Central waits up to a specified timeout for a framebyte from the remote.

If no response is received after the timeout then the EFS command issent to the remote at the next diagnostic address in the poll table. Thediagnostic address of a nonresponding remote will not be removed fromthe poll table here because the remote may be temporarily out of thesynchronization with the outbound framing. Cases such as this will behandled after the user data are brought online. After a reference isestablished in the central, the remaining remotes are aligned to thisreference. This is done by sending the SAP command to a remote causingthe remote to send one frame with the framing pattern in the first frameslot followed by all marks as previously described. If a frame byte isreceived the Central measures how many bit and byte times this is offfrom the reference receive framing. This value is sent in an adjustmessage to the remote for it to use to update its transmit framing. Whenthe remote receives the adjust message it adjusts its transmitter bythat amount, sets its "in-sync" flag and transmits all marks in theframe. After the Central has transmitted the adjust message the Centralis finished with the synchronization session for that remote.

After the Central has attempted to synchronize all the remote stationsfrom the poll table it will then enable user data operation. This isdone by sending a global enable command to the remotes. This enables theremote stations with their "in-sync" flag set to transmit multiportframing and control and data in multiport time slots of the multiportframe. After that point, the remaining diagnostic addresses can bepolled along with the ones in the poll table. This allows user data tobe enabled before having to poll all 254 diagnostic addresses.

The objective is to bring user data back online as soon as possibleafter a condition requiring resynchronization, i.e., line slips, powerfailures etc. To minimize this time, verification of remote'ssynchronization is not performed during the synchronization session inthe preferred embodiment, but this is not to be limiting. Likewisechecking for units that do not appear in the poll table is not performedduring the synchronization session. Both of these tasks are performedafter user data are brought online. The typical case would be that allunits were properly synchronized and there are no remote units on thenetwork that are not in the poll table. Only in the exceptional casewill the user data have to be disrupted to do further synchronization ofa remote. This methodology should minimize user down time for themajority of situations where resynchronization is required.

While the present invention has been disclosed in connection with asystem operating under the control of a Microprocessor, those skilled inthe art will appreciate that hard wired logic equivalents may also bedevised. In addition, although the preferred embodiment uses a customApplication Specific Integrated Circuit (ASIC) in order to perform someof the functions, this is also not limiting since these functions couldequally well be performed by other hardware, firmware or software baseddesigns.

In addition, although the present invention has been disclosed inconjunction with the commonly available DDS services from AT&T, thetechniques disclosed are equally applicable to other networks usingsimilar digital bridging techniques without regard for service type orprovider. The present invention could event be used, for example, withT1 frames if an appropriate digital bridge were present in the networkso that operation in as sort of data mode were possible. Also, althoughthe present invention has been described using positive logic, similarnegative logic are within the scope of the present invention.

Thus it is apparent that in accordance with the present invention amethod and apparatus that fully satisfies the objectives, aims andadvantages is set forth above. While the invention has been described inconjunction with specific embodiments, it is evident that manyalternatives, modifications and variations will become apparent to thoseskilled in the art in light of the foregoing description. Accordingly,it is intended that the present invention embrace all such alternatives,modifications and variations as fall within the spirit and broad scopeof the appended claims.

What is claimed is:
 1. A method for use in a digital data networkwherein at least one digital bridging device combines digital data bitsfrom a plurality of remote stations into a composite signal fortransmission to a central station, comprising the steps of:providingsaid central station with a plurality of central data ports; providingeach of said remote stations with at least one remote data port; therebeing at least one said remote data port for receiving said data bitsfor transmission to each one of said central data ports; at least two ofsaid remote data ports being operable for receiving said data bits fortransmission to a same one of said central data ports; arranging at saidremote stations said data bits from said remote ports into reoccurringfixed size frames with each one of said frames having a plurality oftime slots; there being at least one of said time slots assigned totransmit said data bits to each of said central ports; said assignmentof time slots to said central ports being the same at each of saidremote stations; initially aligning said frames from different saidremote stations by adjusting transmission times from at least one ofsaid remote stations so as to cause said frames from said differentremote stations to arrive at said central station simultaneously;transmitting said aligned frames from said remote stations to saidcentral station; after initially aligning said frames, monitoring todetermine if said inbound frames from said remote stations remain intime alignment; and if said inbound frames from said remote stations donot remain in time alignment, further aligning said frames fromdifferent said remote stations by adjusting transmission times from atleast one of said remote stations so as to again cause said frames fromsaid different remote stations to arrive at said central stationsimultaneously.
 2. The method of claim 1 wherein said step of monitoringfurther includes the steps of:(a) selecting one of said remote stationsfrom a polling list; (b) polling said remote station from said centralstation to determine if said remote station is properly aligned; (c) ifsaid remote station is properly aligned, selecting a next said remotestation from said polling list and repeating step (b) for said nextremote station; and (d) if said remote station is not properly alignedin step (b), proceed with said step of further aligning said frames tocorrect alignment of said remote station and then going to step (a). 3.The method of claim 2 wherein said step of arranging said data bits intosaid frames further includes the step of defining at least one time slotin each of said frames for responses by said remote stations to saidpolling from said central station so that the transmission of said databits from said remote stations is not interrupted by said polling. 4.The method of claim 2 wherein said step (d) comprises determining if aone bit timing slip has occurred and adjusting said alignment by saidone bit timing.
 5. The method of claim 4 wherein said determining stepis performed by transmitting a space surrounded by marks in apredetermined position in a frame and determining if said space ismisplaced by one bit time when received.
 6. The method of claim 1wherein said step of arranging said data bits into said frames includesproviding at least one time slot for transmission of polling responsesfrom said remote stations, andwherein said step of monitoring furtherincludes the steps of:(a) selecting one of said remote stations from apolling list; (b) transmitting a command to said selected remote stationrequesting an acknowledgement response be transmitted by said selectedremote station to said central station in said time slot for saidpolling responses; and (c) listening at said central station for saidacknowledgement response from said selected remote station, wherebyreceipt of said acknowledgement response by said central stationverifies, without interrupting said data bits contained in the othersaid time slots, that said selected remote station remains in timealignment.
 7. The method of claim 6 further comprising repeating saidsteps (a), (b), and (c) for each of said remote stations on said pollinglist.
 8. The method of claim 7 further comprising repeating more thanonce said steps (b) and (c) for a given one of said remote station notproviding said acknowledgement response.
 9. The method of claim 6further comprising, if said acknowledgement response is not received insaid step (c), proceeding with said step of further aligning saidframes, said step of further aligning said frames including the stepsof:commanding all the remote stations to transmit an idle signal, andcorrecting the alignment of said selected remote station.
 10. The methodof claim 9 further comprising, prior to said step of further aligning,the steps of:transmitting a poll message from said central station tosaid selected remote station if said acknowledgement response is notreceived by said central station in error free form, said poll messageincluding an address of said selected remote station; at said selectedremote station, detecting said address; and transmitting a sequence ofspaces from said selected remote station over said network, saidsequence of spaces being positioned in time to overlap any markstransmitted by other said remote stations,whereby said sequence ofspaces is received by said central station regardless of lack of saidtime alignment and thereby confirms that said selected remote station ison line.
 11. The method of claim 10, wherein said sequence of spacesoverlaps a time designated for use by a frame synchronization pattern.12. The method of claim 11, wherein said sequence of spaces is at leastas long as the length of said frame.
 13. The method of claim 1 whereinsaid step of further aligning said frames includes the steps ofcommanding all of said remote stations to enter an idle state;having agiven one of said remote station transmit a signal; measuring adifference in time between receipt of said transmitted signal and areference time; and adjusting a transmission time for transmissions fromsaid given one of said remote stations so that said measured differencein time is eliminated.
 14. The method of claim 13 further comprising thestep of:commanding said given one of said remote stations to return tosaid idle state after said step of further alignment.
 15. The method ofclaim 14, further comprising the step of:commanding said given one ofsaid remote stations to leave said idle state and transmit user dataafter all of said remote stations have been aligned.
 16. The method ofclaim 13, wherein said step of initially aligning said frames includesthe steps of commanding a given one of said remote stations to transmita signal; measuring a difference in time between receipt of saidtransmitted signal and a reference time; and adjusting a transmissiontime for transmissions from said given one of said remote stations sothat said measured difference in time is eliminated.
 17. The method ofclaim 13, further comprising the step of setting an "in-sync" flag to apredetermined value in said given one of said remote stations toindicate that synchronization has been established.
 18. The method ofclaim 1 wherein said step of further aligning includes the steps of:atsaid central station, issuing a global command for all remote stationsto transmit marks in each frame location; commanding a first remotestation to transmit a frame containing a predetermined pattern;establishing a reference time at said central station based upon time ofreceipt of said predetermined pattern; commanding a next remote stationto transmit said predetermined pattern; measuring a relative delay inreceiving said predetermined pattern frame from said next remotestation; commanding said next remote station to adjust its transmissiontime by an amount which causes transmissions from said first and nextremote stations to arrive at said central station in time alignment. 19.The method of claim 18, wherein said first remote station is selectedaccording to its position on a polling list.
 20. The method of claim 1,further comprising the step of coupling each of said central ports to adifferent one of a plurality of data applications, andat least said dataapplication coupled to said central port having said at least two remoteports transmitting data thereto comprising a multipoint dataapplication.
 21. The method of claim 1, wherein said at least two saidremote data ports receiving data for transmission to the said same oneof said central data ports including said at least two remote data portsbeing disposed at different said remote stations, andsaid assigned timeslots in said frames to each of said central data ports resulting in allsaid remote data ports associated with said same one of said centralport transmitting said data bits in the same said time slots, wherebythe corresponding remote data ports at each remote station are assignedthe same time slots.
 22. The method of claim 1, wherein said digitalbridging device uses a logical AND operation and said step of providingincluding providing at least two of said remote data ports which receivesaid data bits for transmission to each one of said central data ports,andeach of said at least tow said remote data ports belonging todifferent ones of said remote stations.
 23. The method of claim 1,wherein said step of providing includes providing at least one of saidremote stations with at least two of said remote data ports forreceiving said data bits for transmission to different said centralports, andeach of said central ports being associated with one of aplurality of data applications.
 24. A digital data communication systemfor a digital network wherein digital data bits are transported betweena plurality of locations, each said data bit lasting for a predeterminedbit time, comprising:a central station coupled to said network, saidcentral station including a plurality of central data ports, each ofsaid central data ports being associated with a different one of aplurality of independent data applications: a plurality of remotestations coupled to said network, each of said remote stations having atleast one remote data port associated with one of said dataapplications, the number of said remote data ports being greater thanthe number of said central data ports, there being a least one of saidremote data ports associated with each of said data applications; saidremote stations each including framing means for arranging said databits from said at least one remote data port into a plurality oftime-division multiplexed frames for transmission through said digitalnetwork to said central station, each of said frames having a pluralityof time slots with at least one of said time slots being assigned toeach of said data applications; alignment means for initially aligningsaid frames transmitted by said plurality of remote stations so to causesaid frames to arrive at said central station simultaneously byadjusting transmission times from at least one of said remote stationsby an integer number of said bit times; means for monitoring alignmentof said frames to determine if said frames from said remote stationsremain in time alignment after initial alignment of said frames; andsaid alignment means, if said frames from said remote stations do notremain in time alignment, being operable for further alignment of saidframes from different said remote stations by again adjustingtransmission times from at least one of said remote stations so as tocause said frames from said different remote stations to arrive at saidcentral station simultaneously.
 25. The digital data communicationsystem of claim 24 wherein said means for monitoring furtherincludes:(a) selection means for sequentially selecting each of saidremote stations from a polling list; and (b) polling means for pollingeach of said selected remote stations from said central station todetermine if said selected remote station is properly aligned prior toselection of a next said remote station from said polling list by saidselecting means; andsaid alignment means providing said furtheralignment if said selected remote station is not properly aligned tocorrect alignment of said selected remote station prior to selection ofa next said remote station from said polling list by said selectingmeans.
 26. The digital data communication system of claim 25 whereineach of said frames further includes at least one time slot forresponses by said remote stations to said polling from said centralstation so that the transmission of said data bits from said remotestations is not interrupted by said polling.
 27. The digital datacommunication system of claim 25 wherein said polling means includesone-bit slip determining means for determining if a one bit timing sliphas occurred and adjusting said alignment by said one bit timing. 28.The digital data communication system of claim 27 wherein said one-bitdetermining means is operable for transmitting a space surrounded bymarks in a predetermined position in an outbound frame sent from saidcentral station and for determining if said space is misplaced by onebit time when received.
 29. The digital data communication system ofclaim 24 wherein said framing means being further operable for providingat least one time slot for transmission of polling responses from saidremote stations and wherein said monitoring means further includes:(a)selecting means for selecting one of said remote stations from a pollinglist; (b) polling means for transmitting a command to said selectedremote station requesting an acknowledgement response be transmitted bysaid selected remote station to said central station in said time slotfor said polling responses; and (c) listening means for listening atsaid central station for said acknowledgement response from saidselected remote station, whereby receipt of said acknowledgementresponse by said central station verifies, without interrupting saiddata bits contained in the other said time slots, that said selectedremote station remains in time alignment.
 30. The digital datacommunication system of claim 29, wherein said polling means transmitssaid command again for said remote station not providing saidacknowledgement response after transmitting said command a first time.31. The digital data communication system of claim 29 wherein saidalignment means, if said acknowledgement response is not received,includes means for commanding all the remote stations to transmit anidle signal; and means for correcting the alignment of said selectedremote station.
 32. The digital data communication system of claim 31further comprising:mean for transmitting, prior to said further aligningby said alignment means, a poll massage from said central station tosaid selected remote station if said acknowledgement response is notreceived by said central station in error free form, said poll messageincluding an address of said selected remote station; means, at saidselected remote station, for detecting said address; and means, at saidselected remote station, transmitting a sequence of spaces to saidcentral station over said network, said sequence of spaces beingpositioned in time to overlap any marks transmitted by other said remotestations,whereby said sequence of spaces is received by said centralstation regardless of lack of said time alignment and thereby confirmsthat said selected remote station is on line.
 33. The digital datacommunication system of claim 32 wherein sad sequence of spaces overlapsa time designated for use by a frame synchronization pattern.
 34. Thedigital data communication system of claim 33 wherein said sequence ofspaces is at least as long as the length of said frame.
 35. The digitaldata communication system of claim 24 wherein said alignment means, ifsad frames do not remain in alignment, is operable for commanding all ofsaid remote stations to enter an idle state;having a given one of saidremote station transmit a signal; measuring a difference in time betweenreceipt of said transmitted signal and a reference time; and adjusting atransmission time for transmission from said given one of said remotestations so that said measured difference in time is eliminated.
 36. Thedigital data communication system of claim 35 further comprising meansfor commanding said given one of said remote stations to return to saididle state after said further alignment by said alignment means.
 37. Thedigital data communication system of claim 36 further comprising meansfor commanding said given one of said remote stations to leave said idlestate and transmit user data after all of said remote stations have beenaligned by said alignment means.
 38. The digital data communicationsystem of claim 35 wherein said alignment means, when initially aligningsaid frames, is operable for commanding a given one of said remotestations to transmit a signal;measuring a difference in time betweenreceipt of said transmitted signal and a reference time; and adjusting atransmission time for transmissions from said given one of said remotestations so that said measured difference in time is eliminated.
 39. Thedigital data communication system of claim 35 further comprising thestep of setting an "in-sync" flag to a predetermined value in said givenone of said remote stations to indicate that synchronization has beenestablished.
 40. The system of claim 24 wherein at least one of saidremote stations includes a plurality of remote data ports, at least twoof said plurality of remote data ports of said at least one remotestation being associated with different ones of said plurality of dataapplications.